1. Field of the Invention
The present invention relates to dual-access test fixtures. More particularly, the present invention relates to dual-access test fixtures for testing printed circuit boards without requiring the insertion or the removal of peripheral PC cards to or from socket connectors.
2. Description of the Related Technology
A test fixture is an apparatus designed for mounting and interfacing a printed circuit board (PCB) to be tested by a test and analysis apparatus, otherwise referred to as a board tester. In general, there are two basic types of testers, in-circuit testers for identifying or verifying electrical shorts, open-circuits, component tolerances and IC chip "clocking," and functional testers for verifying the appropriate functional operation of the PCB. An example of an in-circuit tester is the GENRAD 2272/2282 board tester. The present invention, however, is primarily concerned with fixtures for testing the functional operation of PCBs.
The purpose of a dual-access test fixture is to access test points or pads simultaneously on the top and bottom sides of the PCB under test. As the size of PCBs get smaller and become more densely populated with components, the test pads on the PCB consequently decrease in size. These smaller test pads require greater accuracy and much tighter tolerances of the fixture apparatus, since otherwise good PCBs would fail because of misalignment of the probes or pins with the test pads for electrical interfacing. Furthermore, as the PCBs under test become more sophisticated, it is desirable and often necessary to test separate circuits on the bottom and top sides of the PCB. Several dual-access test fixtures have been implemented to solve some of the ever-increasing alignment problems, but many alignment problems still exist, especially due to the advent of smaller test pads.
The existing test fixtures have several deficiencies causing an inordinate percentages of failures. One major problem is the use of electrical wires in one form or another to complete electrical connections. The wires are typically in the form of cable harnesses, where long, individual wires are used for each test connection, or ribbon cable coupled between the top and bottom fixtures and mounted to corresponding connectors.
Many false signal and noise problems arose when using wire or ribbon cables to electrically connect test pins used to contact the test pads of the PCB under test. The wires add stray capacitance or inductively couple signals in nearby test wires, causing electrical noise and false signals. This problem has not been previously solved, so that the only method to achieve an accurate test on a high speed PCB was to reduce the frequency of the test. Frequency reduction is undesirable, since the test is not completely reliable at slower speeds. The most accurate and reliable test is performed at the true clock frequency.
U.S. Pat. No. 5,157,325 ('325), issued Oct. 20, 1992 to Murphy, discloses an existing version of "wireless" fixture technology, which is hereby incorporated by reference. In the so-called wireless technology of the '325 patent, specially designed test probes comprising doubled-ended pogo pins were mounted to a probe plate, where one side of the test pins would electrically contact or engage the test pads of the PCB under test, and where the opposite end of the test pins would electrically engage test pads on the surface of an interface PCB (IPCB). Two probe plates, each mounted with pogo pins, were provided on either side of the PCB under test. The other ends of the pogo or test pins on the bottom probe plate would electrically engage test pads on a bottom IPCB. The bottom IPCB also had rows of input/output (I/O) pads on its bottom side to electrically interface with I/O pins of the tester unit itself. Corresponding test pads formed on the upper and lower surfaces of the bottom IPCB were electrically connected together through conductive traces on and within the bottom IPCB. Thus, electrical connections were made between the test pads located on the bottom side of the PCB under test and the tester through the pogo pins on the bottom probe plate and the bottom IPCB.
On the top side of the PCB under test, the top probe plate was mounted with similar double-ended pogo pins, which were used in a similar manner to electrically engage test pads on the top side of the PCB under test to corresponding test pads on a top IPCB. The test pads on the top IPCB were electrically connected to a connector mounted on the top IPCB using routed traces on the top IPCB. One end of a ribbon cable was plugged into the connector, and the other end of the ribbon cable was plugged into a similar connector mounted on the bottom IPCB. The test fixture apparatus of the patent '325 was not a true wireless system due to the use of the ribbon cable. The ribbon cable also caused other problems. For example, when closing the fixture for testing, the ribbon cable would often get caught between the top and bottom frames, and would consequently break and need replacement. Worse yet, the internal wires would often be severed, resulting in a hidden defect which was very difficult to detect. Further, breakage of the ribbon cable sometimes caused additional electrical shorts.
The previous dual-access wireless fixture described in the '325 patent used a handler having a separate top access "cassette" containing the top fixture assembly, and a separate bottom frame containing the bottom fixture assembly. The handler caused severe alignment problems resulting in an inordinate number of failures, damage to the PCB under test and also damage to the fixture itself. In particular, when using the old handler in the dual-access application, the top cassette was able to shift or slide relative to the bottom frame before vacuum was applied, causing misalignment and broken test probes. Although guide pins and bushings were used, they were not pre-aligned before vacuum was applied. When vacuum was applied, the top access cassette was forced into alignment while the test pins were in contact with the test pads of the PCB. Such forced shifting and alignment reduced the lifetime of the guide pins and bushings, and the test pins and test pads. Several adjustments and test cycles were likely for each test, resulting in greater test cycle times. The necessary adjustments sometimes involved manually shifting the upper cassette to force alignment, often resulting in damage to the PCB under test, the test pins or other parts of the fixture.
U.S. Pat. No. 5,500,606 ('606), issued on Mar. 19, 1996 to Holmes, incorporated herein by reference for all purposes, discloses a wireless dual-access test fixture. The Holmes test fixture has added features that ensure proper alignment of the PCB under test relative to the bottom of the test fixture. In Holmes, the top access or top fixture comprises a top IPCB, an alignment plate, a spacer plate and a top probe plate mounted with the same or similar double-ended pogo pins as used in other wireless techniques. The top probe plate includes a first set of test pins, having first ends which are aligned to electrically engage the printed circuit board (PCB) under test when a vacuum is applied. The other ends of the top probe plate test pins are aligned to electrically engage test pads located on the top IPCB. The top probe plate also includes a second set of pogo pins, called transfer pins, having one end for electrically engaging transfer pads located on the top IPCB. The top IPCB is routed with electrical traces to connect the test pads to the transfer pads of the top IPCB.
The '606 patent also discloses a bottom fixture that includes a bottom IPCB, an alignment plate and a bottom probe plate mounted together. One side of the bottom IPCB includes input/output (I/O) pads for electrically engaging the I/O pins of the tester apparatus. The other side of the bottom IPCB includes test pads for electrically interfacing with the test pads on the bottom side of the PCB under test, and transfer pads for electrically interfacing with the test pads on the top of the PCB under test through transfer pins. The bottom probe plate includes a first set of test pins, each having one end for electrically engaging the test pads on the bottom IPCB, and another end for electrically engaging corresponding test pads located on the bottom side of the PCB under test. The bottom probe plate also includes a second set of pogo pins, or transfer pins, or test pins, each having one end for electrically engaging the transfer pads on the bottom IPCB, and another end for electrically engaging the transfer pins of the top probe plate. According to Holmes, all of the test pads on the top and bottom side of the PCB under test are electrically interfaced through the test pins, transfer pins and top and bottom IPCB's to the I/O pins of the tester without the use of wires. This substantially reduces false signals due to stray capacitance or inductive coupling, allowing the test to be performed at higher frequencies. Furthermore, since the tests are more accurate, a significant increase in the percentage of good PCBs that pass is achieved.
Another major problem in the art is the failure of the PCB before, during, or after "functional" testing. PCBs normally undergo three main stages of testing: continuity, functional and final. The first test is the continuity test. Continuity testing merely checks to see that there are no "shorts" or other types of mistakes that may damage other parts of the PCB or the test equipment used during subsequent testing. The second test is the functional test wherein electrical signals are input into the PCB and measurements taken to see that the output is consistent with the input. Final testing comes after all of the components are inserted into the PCB and ensures that the customer gets a quality product that is fully operative.
During the functional test, a PCB is placed on a jig and various test cards (such as I/O and memory cards) are inserted into the connector slots. The test cards are tested beforehand and are known function correctly. By using the known functional test cards, any incorrect output responses can be attributed to the PCB under test. After the functional test is complete, the test cards are removed from the PCB and used for subsequent functional tests on other PCBs. Other cards are then inserted into the PCB's connector slots per customer specifications. Unfortunately, the process of inserting and removing cards from connector slots can break or damage the connectors, causing an otherwise good PCB to fail.
It is desirable, therefore, to provide a dual access fixture without signal degrading long wires and provide functional testing without the insertion or removal of test cards. Eliminating the test card insertion/removal process would speed the testing process, reduce stress and breakage of the connector slots, which would improve the reliability of the product. Elimination of the test card insertion/removal process would eliminate the need for a skilled technician to perform the test and enable a less skilled worker to perform the same task.